C# 8+可空引用类型静态分析能否判定两个字段至少一个非空？

你提到的场景是C#可空静态分析的典型约束场景：默认情况下分析器不会自动推导多个字段间“至少一个非空”的互斥逻辑，但可以通过以下几种方式让分析器正确识别目标变量非空。

首先你提供的代码格式化后如下：

class ThereCanBeOnlyOneButAtLeastOne
{
    readonly Thing? knownThing;
    readonly ThingFactory? factory;
    public ThereCanBeOnlyOneButAtLeastOne(ThingFactory factory) => this.factory = factory;
    public ThereCanBeOnlyOneButAtLeastOne(Thing knownThing) => this.knownThing = knownThing;

    public OtherThing CalculateSomething()
    {
        var theThing = knownThing??factory?.Get();
        // 静态分析此时判定theThing为Thing?可空类型
    }
}

实现判定的三种可行方案 #

方案1：调整代码结构封装非空逻辑（最稳妥，无版本依赖） #

将获取实例的逻辑封装为非空只读属性，显式告知分析器返回值必然非空：

class ThereCanBeOnlyOneButAtLeastOne
{
    readonly Thing? knownThing;
    readonly ThingFactory? factory;
    public ThereCanBeOnlyOneButAtLeastOne(ThingFactory factory) => this.factory = factory;
    public ThereCanBeOnlyOneButAtLeastOne(Thing knownThing) => this.knownThing = knownThing;

    // 封装后的非空属性
    private Thing TheThing
    {
        get
        {
            if (knownThing is not null) return knownThing;
            // 逻辑上factory必然非空，加!抑制单路径告警
            return factory!.Get();
        }
    }

    public OtherThing CalculateSomething()
    {
        // 直接使用TheThing，天然为非空类型
        var theThing = TheThing;
        // 其余逻辑
    }
}

方案2：用内置特性显式声明构造函数约束（适配.NET 5+） #

使用System.Diagnostics.CodeAnalysis命名空间下的MemberNotNull特性，显式告知分析器每个构造函数执行完成后对应字段必然非空：

using System.Diagnostics.CodeAnalysis;

class ThereCanBeOnlyOneButAtLeastOne
{
    readonly Thing? knownThing;
    readonly ThingFactory? factory;
    
    [MemberNotNull(nameof(factory))]
    public ThereCanBeOnlyOneButAtLeastOne(ThingFactory factory) => this.factory = factory;
    
    [MemberNotNull(nameof(knownThing))]
    public ThereCanBeOnlyOneButAtLeastOne(Thing knownThing) => this.knownThing = knownThing;

    public OtherThing CalculateSomething()
    {
        // 分析器已识别两个字段至少一个非空，无需写?.，表达式返回非空类型
        var theThing = knownThing ?? factory.Get();
        // 其余逻辑
    }
}

方案3：强制抑制告警（适合临时快速处理） #

如果不想调整现有代码结构，可以直接在表达式末尾加!，显式告知分析器该值逻辑上不可能为空：

var theThing = knownThing??factory?.Get()!;

内容的提问来源于stack exchange，提问作者Chris F Carroll